In certain markets, an engine control module (ECM) needs to know how long the engine in a vehicle has been turned off. For example, onboard diagnostic (OBD) regulations in certain countries may require that the ECM perform cold sensor rationality checks after a certain time period (e.g., 8 hours) of cold soak (i.e., the amount of time between the engine turn off time and the engine start time). In order to determine whether or not to perform the rationality checks, the ECM needs to know the amount of time that has elapsed since the engine was turned off.
One approach to providing the ECM with this information is to include a real time clock (RTC) within the ECM circuitry. Because the RTC needs to run even when the engine is off, a battery is required to power the RTC. If the battery is provided within the ECM, the useful life of the ECM may be reduced since a failure in the battery, or a depletion of the battery, may require that the entire ECM be replaced or serviced. If the battery is external to the ECM, an additional harness may be required, which adds complexity, bulk, and cost to the ECM. Adding an RTC and battery to an ECM not only results in an increase in cost and complexity in markets or for applications that require cold soak tracking, but such cost and complexity is completely excessive in markets or for applications where cold soak tracking is not required or necessary.
Another known approach is to obtain real time information from another component with an RTC by using the vehicle's data bus and a serial data connection of the ECM. For example, the Society of Automotive Engineers (SAE) J1939 is the vehicle bus standard used for communication and diagnostics among vehicle components. The ECM may rely on another module connected to the data bus to provide real time information, which the ECM can then use to obtain cold soak information. However, given the varied configurations of different vehicles, there may be no guarantee that a particular vehicle will have a module that can provide the proper or compatible connections for transmission of real time information to the ECM.